In an extremely rare move, North Korea sent a letter of protest to the U.S. House of Representatives over its new package of tougher sanctions.

The committee of the North’s Supreme People’s Assembly denounced the sanctions as “the most heinous act against humanity that not only infringes upon the sacred sovereignty of (North Korea) but also arbitrarily violates universal principles of sovereign equality and non-interference in the internal affairs of other countries which run through the United Nations Charter.”

While it’s not unusual for Pyongyang to condemn moves to censure it by Washington, a direct protest to Congress is uncommon.

Since North Korea and the United States have no formal diplomatic relations and virtually no official channels of communication, it was not immediately clear how the letter was dispatched — if it was sent by mail, email — or even how it was addressed. The was also no immediate word on whether it had been received.

The report, carried by the North’s Korean Central News Agency, said the letter of protest was sent Friday.

The House on May 4 voted overwhelmingly, 419-1 in favor of the sanctions that target North Korea’s shipping industry and its use of “slave labor” — goods produced by forced labor. Before the sanctions can be implemented, the U.S. Senate will need to approve them, a procedural point apparently lost on North Korea’s leadership USA Today reports.

It also requires that the Trump administration report to Congress within 90 days on whether North Korea should be reinstated on the government’s state sponsors of terror list Fox News reports. Such a designation would trigger more sanctions, including restriction on U.S. foreign assistance.

Adm. Harry Harris Jr., the top American military officer in the Pacific, has warned lawmakers that it’s a question of when, not if, Pyongyang successfully builds a nuclear-tipped missile capable of striking the U.S.

Rep. Thomas Massie, R-Ky., was the sole member to vote against the measure. The Senate will take it up next.

The bipartisan legislation is aimed at thwarting North Korea’s ambitions by cutting off access to the cash the regime needs to follow through with its plans.

Specifically, the bill bars ships owned by North Korea, or by countries that refuse to comply with U.N. resolutions against it, from operating in American waters or docking at U.S. ports. Goods produced by North Korea’s forced labor would be prohibited from entering the United States, according to the legislation.

“The U.S. House of Representatives should think twice,” the foreign affairs committee of the North’s Supreme People’s Assembly said in its letter. “As (the House) enacts more and more of these reckless hostile laws, the DPRK’s efforts to strengthen nuclear deterrents will gather greater pace, beyond anyone’s imagination,” it added, referring to the Democratic People’s Republic of Korea, the country’s formal name.